There is an ongoing desire to reduce atmospheric emissions from chemical plants, and particularly methanol emissions associated with ammonia plants. Reducing such methanol emissions has become critical for both new units and existing units undergoing revamps.
With reference to FIG. 1, in the prior art synthesis gas generation unit 10, such as in an ammonia or hydrogen plant, a hydrogen-rich stream 12 is supplied from a low temperature shift converter (not shown). The low temperature shift catalyst in the converter is typically used to improve shift reaction conversion of carbon monoxide and water to carbon dioxide (CO.sub.2) and hydrogen. This service typically employs a copper-based catalyst which under typical conditions of operation supports some formation of by-products such as methanol from the reactants which are present. Downstream of the shift section, the process stream 12 is cooled in cooler 14 to condense water which is separated from the gas in knock-out drum 16 to form condensate stream 18 and overhead gas stream 20. The condensed process condensate which has a typical methanol content of 500-1000 ppmw is sent by pump L9 to a condensate stripper 22 after heating in condensate stripper feed/effluent heat exchanger 24. Fresh steam is supplied in line 26 to strip contaminants such as ammonia, methanol and higher alcohols and CO.sub.2 from the condensed process condensate in condensate stripper 22. Steam containing the contaminants is recovered overhead via line 28 and supplied to a steam reformer (not shown) via line 30 along with steam by-passing the condensate stripper 22 via line 32. Stripped condensate is recovered as a bottoms stream from condensate stripper 22 via line 34 and can be polished offsite or otherwise processed.
Methanol present in the process gas in line 20 is sent to a purification unit 36 for removal of CO.sub.2 and/or other non-desirable components in the syngas product. The purification unit 36 is typically an absorber-stripper system or a mole sieve system such as a pressure-swing adsorption (PSA) unit. Purified syngas is obtained in line 38. The methanol comes out in a CO.sub.2 -rich overhead product stream 40. In many cases, at least a part of this CO.sub.2 stream 40 is vented to the atmosphere along with any methanol which may be present therein.
It would be desirable to have available a way of reducing the methanol emissions in the CO.sub.2 from the purification unit 36. Ideally, the means for reducing the methanol emissions would minimize additional equipment requirements, would have a minor impact on plant energy consumption, and would not produce solid contaminants which require disposal. Conventional methanol reduction technology such as end-of-pipe catalytic reactors, or alternatively refrigerating the raw syngas to increase methanol separation in knock-out drum 16, do not meet these criteria. The end-of-pipe catalytic reactor requires a blower, a heater (for start-up purposes) and an oxidation reactor, and produces spent catalyst which must be disposed of. Refrigerating the raw syngas would require refrigeration equipment and severe power consumption. Therefore, a need exists for an acceptable way of reducing the methanol emissions.